Self-recovering current limiter

ABSTRACT

A self-recovering current limiter features an outer tubular casing of stainless steel or brass being constructed with spaced flanges provided on the interior surface inside each of the ends thereof and an inner tubular member composed of first and second electrodes and a separating axially aligned insulator tube of beryllia porcelain or alumina porcelain having corresponding flanges being larger in diameter than the inner diameters of the flanges of the outer tube. A glassy material of mica composition is packed between the inner tube and the outer tube and an alkali metal is disposed in an elongate axial channel formed in the inner tube for normally providing conductivity between the first and second electrodes, but upon being vaporized by a shortcircuiting current providing a high resistance to limit current flow therethrough.

Inoue et al.

[451 July 17, 1973 SELF-RECOVERING CURRENT LIMITER Inventors: Takeo Inoue, Tarumi-ku, Kobe-shi,

Hyogo-ken; Takashi Shlrazawa, Uosaki Higashinada-ku, Kobe-shi, Hyogo-ken, both of Japan Mitsubishi Denki Kabushika Kalsha, Toyko, Japan Filed: Dec. 1 6, 1971 Appl. No.: 208,568

Assignee:

Foreign Application Priority Data Dec. 16, I970 Japan 45/l 1268] US. Cl 337/119, 337/21, 337/121, I I 337/158 Int. Cl. 01h 87/00 Field of Search 337/21, 158, 159, 337/114,118,119,161,164,121

Relerences Cited UNITED STATES PATENTS 2/1972 Yamagata ct al 337/158 X 3,513,426 5/1970 Inowe et al. 337/159 UX Primgry ExaminerBernard A. Gilheany Assistant Examiner-A. T. Grimley Attorney-Norman F. Oblon et al.

[ 57] ABSTRACT A self-recovering current limiter features an outer tubular casing of stainless steel or brass being constructed with spaced flanges provided on the interior surface inside each of the ends thereof and an inner tubular member composed of first and second electrodes and a separating axially aligned insulator tube of beryllia porcelain or alumina porcelain having corresponding flanges being larger in diameter than the inner diameters of the flanges of the outer tube. A glassy material of mica composition is packed between the inner tube and the outer tube and an alkali metal is disposed in an elongate axial channel formed in the inner tube for normally providing conductivity between the first and second electrodes, but upon being vaporized by a shortcircuiting current providing a high resistance to limit current flow therethrough.

5 Claims, 9 Drawing Figures PArzmtn mnm SHEU ,1 BF 2 PRIOR ART FIG.3

FIG.-

4 v v v 1 r I (O) 3u- Z 330 FIG. 4

' INVENTORS TAKEO I NOUE TAKASHI SHIRAZAWA ATTORNEYS BY (9mm 5mm W s am/a SELF-RECOVERING CURRENT LIMITER BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates generally to selfrecovering current limiting devices characterized by a closed container holding a metal current limiting material being liquid or solid at ordinary temperatures which provides high impedance when caused to evaporate by a fault or short circuiting current and then recovers initial high electric conductivity when returning to its former liquefied or solidified condition after a specific period of time.

More particularly, however, the invention relates a self-recovering current limiter of simple structure and excellent mechanical strength, and having a high density of an insulating material, which can be prepared without complicated mechanical processing. A

2. Description of the Prior Art The current limiting phenomenon provided by a selfrecovering current limitlng element of the character described is caused by vaporizing a current limiting material such as Na, K, NaK and the like, for example, filling a container, by Joule heat to convert the same to a high-temperature, high-pressure plasma having a remarkably high resistance.

Accordingly, the circuit current is limited to less than a predetermined value, and once the current limit operation is performed, the vaporized metal cools and is liquefied or solidified to recover its original electric conductivity again. In the current limiter, the pressure of the closed container holding the current limiting material is remarkably increased and thus requires a structureof very high mechanical strength.

FIG. 1 is a sectional view of one example of a conventional self-recovering current limiter, wherein the reference numeral 1 indicates an electrode having a circular peripheral flange la and an elongate axial channel 16 as shown in FIG. 2(b), and the reference numeral 2 indicates an insulating tube provided between one edge of the electrode 1 and the bottom of a bore formed in one end of a container 3 having arcuate channel portions in the tubular wall thereof for receiving two plate pieces 3a, shown in FIG. 2(a), near the upper edge.

A second electrode 4 having buffers 7 and 8 disposed inside an elongate through channel therein is threaded externally at one end for being threadably received in the bottom of the container 3.

An insulating material is packed between the electrode 1, the insulating tube 2 and the outer tubular container 3, and the current limiting material is disposed in an axial passage connecting the electrodes 4 and l. A screw cap 9 is provided in the open end of the second electrode 4 and a needle valve 6 is provided in the open end of the first electrode 1.

As stated above, in the conventional current limiter, the electrode 4 is threadably secured to the outer tube, or container, 3. However, accurate processing and high quality parts are required for sealing the high pressure gas which is developed during operation of the device. Accordingly, a high quality control is necessary for each part. Another disadvantage is caused by air existing in the packing portion before packing the insulating materialS therein which remains in the space between the insulating tube 2 and the outer tube 3 in a compressed condition, whereby high density packing of the insulating material 5 is quite difficult to obtain.

Thus, in the past, parts having different mechanical strength, and accordingly being likely to be broken, have been necessarily incorporated in such devices. Also, in the conventional current limiter, the outer tube 3 is connected to the second electrode 4, such that it is also necessary to provide insulation in the assembly and to consider the safety of operation.

Typical conventional self-recovering current limiters holding the current limiting material in a closed channel are disclosed in the U. S. Pat. No. 3, 501, 730 to Toshio Ito et al., the U. S. Pat. No. 3,1 17,207 to Ralph L. Hurtle, the U. S. Pat. No. 3,389,360 to James J. Keenan and the U. S. Pat. No. 3, 389,359 to Lawson P. Harris. These conventional current limiters have various disadvantages as already covered, such as complicated mechanical processing, complicated structure, unsuitable insulation and inadequate mechanical strength.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide an improved self-recovering current limiter having an outer casing which is completely insulated from the electrodes.

It is another object of this invention to provide a selfrecovering current limiter of simple construction having excellent mechanical strength and high density packing of an insulating material which can be prepared without complicated mechanical processing.

The foregoing and other objects are attained according to this invention through the provision of a selfrecovering current limiting device having an outer tubular casing with an interior flange formed just inside each of the ends thereof, an inner tubular member being composed of axially aligned first and second electrodes spaced by an insulating tube and having respective flange portions which are larger in diameter than the inner diameter of the interior flanges of the casing, an insulating material being packed between the inner tubular member and the outer tubular casing, and a current limiting material disposed in a common axial channel of the electrodes and insulating tube to provide electrical conductivity therebetween.

BRIEF DESCRIPTION OF THE DRAWINGS Various other objects, features and attendant advantages of the present invention will be more fully appreciated as the same becomes better understood from the following detailed description when considered in connection with the accompanying drawings, wherein like reference characters designate like or corresponding parts throughout the different views, and in which:

FIG. 1 is a sectional view of one example ofa conventional current limiter;

FIGS. 2(a) and 2(b) are plan views of plate pieces cooperable with an outer casing for forming an interior flange and of the upper electrode of the device shown in FIG. 1, respectively;

FIG. 3 is a sectional view of an embodiment of the current limiter in accordance with present invention;

FIGS. 4(a) and 4(b) are a plan view of plate pieces cooperable with an outer casing for forming an interior flange-and an end view of one of the electrodes shown in the FIG. 3, respectively;

FIG. is a sectional view of another embodiment of the current limiter of the present invention; and,

FIGS. 6(a) and 6(b) are plan views of a snap ring for the'outer tube and a snap ring used for the first and second electrodes shown in FIG. 5, respectively.

DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS Referring now to the drawings, and more particularly to FIG. 3, an electrode 1 having a circular flange 1a at one edge thereof and an elongate channel 16 extending axially therethrough is positioned in one end of a casing 3. An insulating tube 2 having an axial through bore 2a is positioned inside the casing 3 on one side of the electrode l and in axial alignment therewith while a flange is provided on the other side by inserting two plate pieces into areuate channels formed in the tubular wall of the casing. The diameter of the flange la of the electrode l is greater than the inner diameter of the flange formed by plates 3a.

A second electrode 4 having a circular flange 4a at one end thereof and a channel 4b axially formed therein is disposed in the other end of the casing 3, and an insulating'material 5 is packed between the interior wall of the casing 3 and the exterior surfaces of the electrodes 1 and 4 and the insulating tube 2.

The current limiter is prepared by mounting the insulating tube 2 on the electrode 4, placing the outer tube 3 around the insulating tube and the electrode, mounting the electrode 1 on the other end of the insulating tube 2, and inserting both sets of plates 3a inside the outer tube 3.

Then, the insulating material is packed, a sealing member 7, O-ring 7a and metallic spring 8 are inserted into the end of the electrode 4, and screw 9 is installed. The long channel 16 of the electrode 1, the throughbore 2a of insulating tube 2 and the channel 4b of the electrode 4 are then filled with a suitable current limiting material by means of a needle valve 6 in the electrode l. The insulating tube 2 used for the embodiment as stated above is preferably a heat-resistant and alkali metal-resistant insulator made of beryllia porcelain, alumina porcelain, or the like. The insulating material preferably is a glassy material-mica composition. The outer tube 3 is preferably made of non-magnetic metallic material, such as stainless steel or high tensile brass, and the electrodes 1 and 4 are preferably made of copper or copper alloy.

The current limiting material filling the channels 16, 2a and 4b is vaporized and provides high pressure such as atmospheric pressure of the order of thousands when current is limited. Accordingly, the pressure is given to the hole 2a of the insulating tube 2 as inner pressure. Even though the sealing valve 7 and the metallic spring 8 in the hole 4b of the electrode 4 is provided as a shock absorbing device, the insulating tube 2 suffers high tensile stress in both the peripheral and axial directions.

The beryllia or alumina porcelain of the insulating tube 2 has a high pressure yield strength of about 100 kg/mm, but a low tensile strength such as about 10 kg/cm. Accordingly, when the tensile stress is produced by this inner pressure, there is a tendency for the insulating tube 2 to be broken, whereby the current limiting material filling the same would be leaked to destroy the functionability of the current limiter.

In accordance with this invention, however, the outer tube 3 is made of metallic material having a coefficient of thermal expansion larger than that of the insulating tube 2 and the insulating material is packed under pres sure and at high temperature. I

Accordingly, high pressure is retained in the insulating tube 2 through the insulating material in the peripheral and axial directions by contraction of the outer tube 3 during the cooling period, whereby the insulating material may be durable to the tensile stress caused by the inner pressure caused by vaporizing the current limiting material. Since the insulating tube 2 and the surrounding easing 3 are formed of materials having different coefficients of expansion, high tensile and compressive forces are avoided, thereby limiting the possibility of structural failure of the device.

Especially, since no sealed portion is provided in the outer peripheral part of the insulating tube, air is not contained in the insulating material. The pressure stress caused by the contraction of the outer tube 3 is applied to the insulating tube 2 without being absorbed by the insulating material 5, so that the pressure stress remaining on the insulating tube 2 is remarkably large to in crease its pressure resistant strength.

FIG. 5 shows another construction of the embodiment shown in FIG. 3. In this case, the flanges on the electrodes are formed by two snap rings 11 being inserted in peripheral grooves and 400 of the electrodes l and 4, respectively, and the flanges on the outer tube 3 are formed by a pair of snap rings 10 being inserted in annular grooves 300 on the interior surface of the outer tube. The snap rings 11 have flanges 11a and are fitted onto the electrodes han asc00ll5 and 4 by spring action, as shown in FIG. 6, being spread apart for placing on the electrodes. The snap rings 10 have flanges 10a extending inwardly therein for being fltted to the outer tube 3 also by spring action, being compressed for placing in the tube, and thereafter expanding into the grooves 300.

In assembling the current limiter of FIG. 5, the electrodes l and 4 are inserted into the casing 3 by passing the flanges 11a of the snap rings 11 through cut-away insert portions provided between the flanges 10a of the snap rings 10, and then the outer tube 3 is turned through 60 so as to engage the flanges 11a of the snap rings 11 with the flanges 10a of the snap rings 10.

As stated above, the current limiter of the present invention has a high density of insulating material packed between the insulating tube 2 and the outer tube 3, without air being trapped therein, and has excellent mechanical strength which is durable to sustain large pressure increases by causing the pressure stress of thermal shrinkage of the outer tube 3 to be relieved by the insulating tube 2, and further has the advantage that no complicated mechanical processing is required because of the simple structure.

Particularly, the current limiter of the present invention has an excellent effect in that the outer tube 3 is entirely insulated from the electrodes, whereby the operation to secure the current limiter is greatly simplitied and is useful in the maintenance of the same.

Obviously many modifications and variations of the present invention are possible in light of the above teachings. It is to be understood, therefore, that within the scope of the appended claims, the present invention can be practiced otherwise than as specifically described herein.

What is claimed as new and desired to be secured by Letters Patent of the United States is:

1. A self-recovering'current limiter comprising:

an outer substantially tubular casing having a pair of said first snap rings;

an insulating material packed between the inner tubular member and the outer tubular casing; and

a self-recovering current limiting material for norradially extending flanges respectively provided on 5 mally providing conductivity between the first and the inner wall inside each edge thereof; second electrodes being disposed in said inner tuan inner tubular member comprising first and second bular member, said material being vaporizable proaxially spaced electrodes and an insulating tube viding a high impedance when heated by a current disposed therebetween in axial alignment thereof predetermined amount.

with, providing an elongate axial channel there- 10 '3. A self-recovering current limiter as set forth in through, said tubular member having a pair of flanges respectively formed on the periphery of each of the electrodes being larger in diameter than the inner diameter of the flanges on said outer tubular casing and being positioned respectively axially inwardly of said pair of flanges on said tubular claim 1, wherein:

said first snap rings are fitted in grooves formed on the inside wall of the outer tubular casing adjacent the ends thereof; and, said second snap rings are fitted in grooves on the outer peripheral walls of the first and second eleccasing; trodes. an insulating material packed between the inner tu- 4. A self-recovering current limiter of claim 3,

bular member and the outer tubular casing; and, wherein:

said first snap rings are formed in a C-shaped configuration having a plurality of spaced flanges extending radially inward thereof,

said second snap rings are formed in a C-shaped configuration having a plurality of spaced flanges extending radially outward therefrom; and,

the arcuate space between the flanges of said first snap rings is larger than the arcuate width of the flanges of said second snap rings, and the arcuate space between the flanges of said second snap rings is larger than the arcuate width of the flanges of said first snap rings, respectively.

I v 5. A self-recovering current limiter as set forth in electrodes and an insulating tube disposed therebeclaim 4, wherein the axis of said outer tubular casing, tween in axial alignment therewith, said tubular the axis of said insulating tube, the axial line of said first member having at least one'flange formed on the 35 snap rings, and the axial line of saidsecond snap rings 7 periphery thereof by at least one second snap ring are aligned with each other.

being larger in diameter than the inner diameter of a self-recovering currentlimiting material for nor- 20,

- mally providing conductivity between the first and second electrodes being disposed in said elongate axial channel, said material being vaporizable and providing a high impedance when heated by a cur rent of predetermined amount. I

2. A self-recovering current limiter comprising:

an outer substantially tubular casing having a pair of radially extending flanges formed on the inner wall thereof by a pair of first snap rings fitted inside said outer tubular casing adjacent both ends thereof,

- respectively;

an inner tubular member comprising first and second 

1. A self-recovering current limiter comprising: an outer substantially tubular casing having a pair of radially extending flanges respectively provided on the inner wall inside each edge thereof; an inner tubular member comprising first and second axially spaced electrodes and an insulating tube disposed therebetween in axial alignment therewith, providing an elongate axial channel therethrough, said tubular member having a pair of flanges respectively formed on the periphery of each of the electrodes being larger in diameter than the inner diameter of the flanges on said outer tubular casing and being positioned respectively axially inwardly of said pair of flanges on said tubular casing; an insulating material packed between the inner tubular member and the outer Tubular casing; and, a self-recovering current limiting material for normally providing conductivity between the first and second electrodes being disposed in said elongate axial channel, said material being vaporizable and providing a high impedance when heated by a current of predetermined amount.
 2. A self-recovering current limiter comprising: an outer substantially tubular casing having a pair of radially extending flanges formed on the inner wall thereof by a pair of first snap rings fitted inside said outer tubular casing adjacent both ends thereof, respectively; an inner tubular member comprising first and second electrodes and an insulating tube disposed therebetween in axial alignment therewith, said tubular member having at least one flange formed on the periphery thereof by at least one second snap ring being larger in diameter than the inner diameter of said first snap rings; an insulating material packed between the inner tubular member and the outer tubular casing; and a self-recovering current limiting material for normally providing conductivity between the first and second electrodes being disposed in said inner tubular member, said material being vaporizable providing a high impedance when heated by a current of predetermined amount.
 3. A self-recovering current limiter as set forth in claim 1, wherein: said first snap rings are fitted in grooves formed on the inside wall of the outer tubular casing adjacent the ends thereof; and, said second snap rings are fitted in grooves on the outer peripheral walls of the first and second electrodes.
 4. A self-recovering current limiter of claim 3, wherein: said first snap rings are formed in a C-shaped configuration having a plurality of spaced flanges extending radially inward thereof, said second snap rings are formed in a C-shaped configuration having a plurality of spaced flanges extending radially outward therefrom; and, the arcuate space between the flanges of said first snap rings is larger than the arcuate width of the flanges of said second snap rings, and the arcuate space between the flanges of said second snap rings is larger than the arcuate width of the flanges of said first snap rings, respectively.
 5. A self-recovering current limiter as set forth in claim 4, wherein the axis of said outer tubular casing, the axis of said insulating tube, the axial line of said first snap rings, and the axial line of said second snap rings are aligned with each other. 